Forms Block: Add hidden input field variation

[SirLouen]

Closes #57610

Why?

One of the most important fields in any form is the hidden type. This PR aims to add this.

How?

Part of the logic and styles were already implemented but not being used. This PR takes advantage of this with the following additions:

1. Advanced Text Control, next to Name to set Value. To some extent this mimics the same behavior as Elementor forms.
2. There was a field template for the component that was targeting the hidden input (but adding hidden in the editor, which made 0 sense). I simply took advantage of it and styled a little bit to resemble the Form Submission Notification block, which happens to have two hidden fields (that only appear on submission). This way, it's clear that there are hidden inputs set in the form, they are selectable without needing to unfold the Document Overview.

Testing Instructions

Very easy to test.

1. Enable the form experiments in Gutenberg > Experiments > Blocks: add Form and Input blocks
2. Create a new post
3. Add a new Form block
4. Add the Hidden Input block inside the form.
5. Add a value (in the right controls Block > Advanced)
6. Save and check the front end
7. Check the HTML code, and look for the hidden field; it should have the value you set.

Screenshots

[github-actions]

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Co-authored-by: SirLouen <sirlouen@git.wordpress.org>
Co-authored-by: bradhogan <bradley2083@git.wordpress.org>

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[SirLouen]

Please don't forget to add

Co-authored-by: dhananjaykuber <dhananjaykuber@git.wordpress.org>

As he also did some effort to make this happen.

[carolinan]

I think we should merge this, but before removing any experimental state, we could reconsider if the Advanced panel is the right place for name and value input.